1. Field of the Invention
The present invention relates to an optical recording medium comprising a substrate having formed thereon a pattern of depressions and projections and a light-transparent layer formed on the substrate, and a method of manufacturing the optical recording medium.
2. Description of Related Art
As typical conventional optical recording media from or into which data is read or written using a light, ones shaped to have a generally disc-like form, so-called optical discs, are well known of which an optical disc substrate is formed from a synthetic resin such as polycarbonate or similar. The optical discs include a read-only type from which a signal already recorded therein can be read, a write-once type into which a signal can be recorded only once, and a rewritable one into which a signal can be recorded more than once.
The read-only type optical disc comprises an optical disc substrate having a pattern of microscopic depressions (so-called xe2x80x9cpitsxe2x80x9d) formed on a surface thereof in the direction of track, and a reflective layer provided on the disc surface on which the pit pattern is formed. Namely, the optical disc has a signal recorded as a pattern of pits formed in the direction of a track. For reading a signal recorded as a bit pattern from this read-only type of optical disc, the disc is irradiated with a reading light from a surface thereof opposite to the surface on which the bit pattern is formed. The reading light is reflected at the reflective layer, and a return light (reflected reading light) is detected to read the signal.
Also in this optical disc, a diffracted light from the surface on which the pits are formed is detected for maintaining the spot of light from a source accurately on the track (so-called tracking).
On the other hand, the optical discs of the write-once and rewritable types comprise each an optical disc substrate having formed thereon concentric or spiral depressions (so-called groove) and projections (so-called lands) formed between adjacent grooves, recording layer provided on the substrate, and a reflective layer provided on the recording layer. In the optical discs of these types, a signal is written on the recording layer formed on the grooves and/or lands. Also in these write-once and rewritable optical discs, pit patterns as additional signals such as disc properties, addresses, etc. are formed between grooves adjacent to each other in the direction of track.
When these optical discs are played, a reading light is irradiated onto the recording layer formed on the lands and/or grooves from on a surface thereof opposite to a surface on which the pits and grooves are formed, thereby forming recording marks on the recording layer. For reading the optical discs of these types, a reading light is irradiated as in the read-only type optical disc, and a return light from the disc is detected.
Also in these write-once and rewritable types of optical discs, a return light reflected from the groove, for example, is detected for the purpose of tracking.
The read-only, write-once and rewritable optical discs (will be referred to as xe2x80x9coptical discxe2x80x9d hereinunder) are formed by an injection melding using a stamper having formed thereon projections and depressions which will provide pits and grooves on the optical disc. More specifically, a synthetic resin such as polycarbonate is injection-molded to form a transparent substrate having pits and grooves formed thereon. A reflective layer, etc. are formed on the transparent optical disc substrate to form an optical disc. In the write-once and rewritable optical discs, a recording layer is formed between the transparent optical disc substrate and the reflective layer.
A recent tendency in this field of art is to decrease the diameter of the spot of light focused on the optical disc in order to attain a higher recording density. Generally, this object can be attained by using a light of shorter wavelength and an objective lens of a larger numerical aperture (NA).
In case the diameter of the spot of light focused on the optical disc is decreased for a higher recording density, however, the transparent optical disc substrate has to be increased in thickness. This is because as the NA of the objective lens is increased, the allowance of an aberration caused by a tilt angle, which the disc surface or plane forms with respect to the optical axis of an optical pickup, is smaller. Also, the thicker a disc portion through which a reading light is transmitted, the larger the aberration due to the tilt angle will become.
Therefore, the distance between a light-incident surface and signal layer of the optical disc is decreased to accommodate the decreased diameter of the spot of light focused on the optical disc.
In the above-mentioned optical disc, however, it is difficult to mold a pattern of depressions and projections on an optical disc substrate having a thickness of 0.3 mm or less, for example. Further, even if the molding is possible, the optical disc substrate is likely to warp, which will result in a difficulty in forming thereon layers such as recording layer, reflective layer, etc. Moreover, if the optical disc substrate having such a thickness, the user will not be able to easily handle an optical disc made from the optical disc substrate.
Therefore, there has so far been proposed an optical disc comprising a transparent disc substrate having a pattern of depressions and projections formed thereon, at least a reflective layer formed on the disc substrate, and a light-transparent layer formed above the reflective layer so that a light of a predetermined wavelength can be incident through the light-transparent layer to read and/or write data from and/or into the optical disc. This type of optical disc will be referred to as xe2x80x9cread-from-rear optical discxe2x80x9d hereinunder for the convenience of description.
In this read-from-rear optical disc, a recording layer is formed between the reflective and light-transparent layers.
In the read-from-rear optical disc, the light-transparent layer can be formed very thin compared with the optical disc substrate having depressions and projections formed thereon. Thus, even with a light of a larger wavelength and an objective lens of a larger NA, read and write of a signal can be made accurately with respect to the optical disc without the above-mentioned problems.
Even in the aforementioned read-from-rear optical disc, however, the transparent optical disc substrate is formed by an injection molding using a stamper having formed thereon a pattern of projections and depressions which will provide pits and grooves on the optical disc substrate. To prepare the stamper, first a photoresist layer is formed over a glass substrate, and a predetermined area of the photoresist layer is exposed to a light, thereby providing a glass master having provided thereon a predetermined pattern of depressions and projections formed from the unexposed photoresist layer remaining on the glass substrate. Next, the glass master is plated with nickel or similar to form a stamper.
Therefore, to form a pattern of pits/grooves and lands on the transparent optical disc substrate with a high accuracy for this read-from-rear optical disc, it is necessary to form a pattern of projections and depressions with a correspondingly high accuracy and also replicate the pattern accurately to the optical disc substrate.
For smaller pits and grooves to attain a higher recording density, it is more difficult to replicate the pattern of projections and depressions on the stamper to the optical disc substrate with a high accuracy. For instance, for microscopic projections formed on one main surface of the transparent optical disc substrate, corresponding microscopic depressions shall be formed on the stamper. Therefore, a resin has to be filled or charged in such microscopic depressions on the stamper.
It is conceivable that when producing an optical disc, a resin of a lower viscosity could be filled into the microscopic depressions with a higher efficiency and a higher accuracy of replication. Actually, however, it is difficult to fully charge such a lower-density resin into microscopic depressions. Namely, it is not possible to mold pits with a high accuracy.
An experiment to prove the above was done as will be discussed below. In the experiment, polycarbonate was used as a resin to be charged into microscopic depressions. It was injection-molded with a mold temperature changed to change the temperature of the resin. The mold temperatures used in this experiment were in three kinds: 120, 125 and 130xc2x0 C.
The injection molding was done using a stamper adapted to form grooves of 0.85 xcexcm in track pitch and 120 nm in depth. The accuracy of replication of the grooves thus molded was measured. The result of the measurement is shown in FIG. 1. In FIG. 1, the vertical axis indicates a groove depth (nm) and the horizontal axis indicates a ratio (%) between groove width in the stamper and track pitch. That is, the horizontal axis in FIG. 1 indicates a ratio in area of the grooves over the stamper.
As seen from FIG. 1, a higher mold temperature provides a higher accuracy of replication. As also known from FIG. 1, however, even at a high mold temperature, the accuracy of replication is lower with a smaller area of the grooves over the stamper. This means that for example, with depressions formed intermittently in the direction of track on the stamper correspondingly to pits on an optical disc substrate, it is difficult to form microscopic pits. Also if the mold temperature is too high, a molded optical disc substrate will warp very much so that the spot of a light such as reading light cannot be just focused even if the projections formed on a stamper have been replicated on the optical disc substrate with a high accuracy of replication. Furthermore, a mold at a high temperature will be expanded itself, which will cause the molding machine to malfunction. If the mold is thermally expanded too much, the moving piece of the mold will incur a so-called galling with a result that the molding machine will possibly damaged. Therefore, the upper limit of the mold temperature can be said to be about 130xc2x0 C.
By the way, the read-from-rear optical disc had better to have recording marks formed on portions of the transparent optical disc substrate that are convex as viewed from the light-transparent layer. Also, the upper surfaces of those projecting or convex portions should preferably be flush with the flat surface of the glass master. In the read-from-rear optical disc, the projecting or convex portions are used as a recording area, thereby increasing the written or read amount of signal and improving the writing and reading characteristics.
In the read-from-rear optical disc, microscopic projections corresponding to the pits on the aforementioned optical disc are formed in line with the portions being convex as viewed from the light-transparent layer, namely, in the direction of recording track. That is to say, the read-from-rear optical disc has additional signals such as disc properties, addresses, etc. written thereon as these microscopic projections.
For a read-from-rear optical disc having formed thereon microscopic grooves and projections for a higher recording density, it is necessary to form the microscopic projections with a higher accuracy. For molding such microscopic projections, however, a stamper having corresponding microscopic depressions formed thereon has to be used. In this case, the accuracy of replication will be considerably lower for the above reasons.
The manufacture of the read-from-rear optical disc is disadvantageous in that the projections formed as additional signals cannot be replicated with a high accuracy, which will lead to a low yield and thus the additional signals could not be read accurately from the corresponding projections not precisely replicated.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing an optical recording medium having a pattern of microscopic depressions and projections and lands formed with a high accuracy of replication, and an optical recording medium manufacturing method capable of accurately forming microscopic depressions and projections on an optical recording medium.
The above object of the present invention can be attained by providing an optical recording medium comprising a substrate and at least a light-transparent layer provided on the substrate, and with respect to which a signal is written and/or read by a light of a predetermined wavelength irradiated from above and through the light-transparent layer;
the substrate having formed thereon projections being convex as viewed from the light-transparent layer and depressions positioned in line with the projections and deep from a same height as the main surfaces of the projections.
In the optical recording medium according to the present invention, the depressions are formed in line with the projections. In the recording medium, the depressions write additional signals, etc. Since formed with a high accuracy of replication, the depressions have each a desired shape. Therefore, the signals can be read accurately and positively from the depressions on this optical recording medium.
The above object of the present invention can also be attained by providing a method of manufacturing the optical recording medium according to the present invention, comprising the steps of:
forming a photoresist layer on a substrate;
exposing a predetermined area of the photoresist layer formed on the substrate to form a plurality of trains of depressions in the photoresist layer;
removing the photoresist layer between the adjoining depression trains to form a master;
plating the master with a metal;
separating the metal coating to form a master stamper;
replicating the master stamper repeatedly an odd number of times to form a mother stamper;
replicating the mother stamper to form a resin substrate; and
forming at least a light-transparent layer on a surface of the resin substrate on which the mother stamper has been replicated.
In the optical recording medium manufacturing method according to the present invention, a plurality of trains of depressions disposed in line is formed and the photoresist layer between the adjoining depressions is removed to form a master. Therefore, the photoresist layer remaining not removed on the substrate is formed as microscopic depressions. Namely, the method of the present invention will form microscopic depressions on the master. Also in this method, since the master stamper is formed by replicating the master, it will have formed thereon a pattern of projections and depressions corresponding depressions and projections, respectively, on the master. Furthermore, in this method, since the mother stamper is formed by replicating the master stamper repeatedly an odd number of times, it will have an inverted one of the pattern of depressions and projections formed on the master stamper. Therefore, the mother stamper will have the same pattern of depressions and projections as that on the master.
Moreover, in this method, since the mother stamper is used to mold the resin substrate, the latter will have replicated thereon the pattern of depressions and projections formed on the mother stamper. Namely, the resin will be applied onto the mother stamper having projections corresponding to the microscopic projections formed on the master. The projections on the mother stamper are replicated to the substrate, resulting in depressions on the transparent substrate.
In the optical recording medium manufacturing method according to the present invention, the mother stamper is formed by replicating the master tamper repeatedly the odd number of times. This means, a stamper or mold formed by replicating the master stamper once is used as a mother stamper, alternatively a stamper formed by replicating the master stamper once is used for a second replication, a stamper formed by the second replication is used for a third replication, and a stamper formed by the third replication is used as a mother stamper. That is to say, the replication of the master stamper is counted as the first replication. A stamper obtained by this first replication is used for a next replication, and a stamper formed by a final odd replication is used as the mother stamper.
In the method of the present invention, grooves may be formed in line with the train of depressions.
In this case, the grooves formed in line with the same train of depressions on the master will be grooves on the mother stamper as well. Therefore, an optical recording medium manufactured by this method will have projections corresponding to the grooves on the mother stamper.
Also, the bottoms of the grooves and depressions should preferably be formed from the substrate.
In this case, since the bottoms of the grooves and depressions are defined by the substrate surface, portions of the resin substrate molded correspondingly to the bottoms of the grooves and depressions will be replications of the substrate surface. Namely, this method will permit to manufacture an optical recording medium having portions formed by replication of the substrate surface.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.